1. Field of the Invention
This invention relates generally to a noncontacting range sensing and control device for controlling the position of an implement relative to a datum used primarily with road construction and earth moving vehicles. The invention relates more particularly to a device and method for avoiding positioning errors due to spurious ultrasonic ranging echoes.
2. Description of the Relevant Art
Motor graders, bulldozers, pavers, milling machines and other road construction and earth moving vehicles are often called upon to produce a graded or paved surface with reference to a pre-defined datum. In areas of new construction, for example, the datum may be defined as a string line supported by stakes placed adjacent to the path to be graded. In areas of reconstruction the datum may be an existing curb or pavement surface.
Various devices have been used on earth moving and paving vehicles to obtain the desired finished surfaces. The grading implement used by such a vehicle is typically positioned by hydraulic cylinders controlled by a positioning control device. A typical positioning control device includes a datum sensor mounted to the implement that senses the position of the implement relative to the datum, and a controller that responds to the sensed implement height and signals the hydraulic cylinders to reposition the implement accordingly.
One such datum sensor is an ultrasonic range sensing device, which operates by transmitting a burst of ultrasound, and then measuring the time required for the burst echo to return from the datum. Ranging bursts are sent out at regular intervals (40 times per second, for example), and each ranging interval results in a time or distance measurement. An example of an ultrasonic range sensing device in the prior art is shown in U.S. Pat. No. 4,733,355, which was issued to Agtek Development Company, Inc., a predecessor of the assignee of the present invention. The disclosure of U.S. Pat. No. 4,733,355 is hereby incorporated by reference.
Using the ultrasonic ranging method, spurious echoes from objects other than the datum may be produced from a ranging burst. Echoes return sequentially with an echo from the closest object returning first. As a rule, the first echo received, after waiting a specified settling time, is assumed to be an echo from the datum and is thus used to determine ranging distance. Echoes received after the first echo are assumed to be from objects other than the datum and are ignored as spurious.
This method works satisfactorily as long as all echoes from previous ranging bursts have diminished sufficiently in amplitude so as not to be detected during a subsequent ranging interval. When spurious echoes from a previous ranging interval have sufficient amplitude to be detected during a later ranging interval, measurement errors can occur. Errors of this type have been observed in conditions where ultrasound bursts bounce back and forth multiple times between the grading or paving vehicle and the surface on which it is grading. Such situations are referred to as "multipath errors."